Optical disks, such as CDs, DVDs and BDs, are made of a transparent resin (although some products are colored within the visible light region) and normally have a thickness of approximately 1.2 min and a diameter of 120 mm, with a central hole of 15 mm in diameter.
FIG. 8 is a diagram showing the structure of a commonly used optical disk 100, where FIG. 8(a) is a plan view and FIG. 8(b) is a sectional view at the arrowed line X-X in FIG. 8(a), each view showing a CD or DVD on the right half and a BD on the left.
An optical disk holds information in its specific layer; in CDs, this layer is located on the side opposite to the readout surface, while DVDs have this layer at a depth of approximately 0.6 mm from the readout surface. In BDs, the information is recorded in a layer 0.1 mm below the readout surface. The information held in the information record layer is read by throwing a laser beam onto this layer through the readout surface and detecting a reflected beam coming from the same layer.
Therefore, if the readout surface of the optical disk is scratched, it is in principle impossible to correctly read information because the laser beam for reading the information and the reflected light coming from the information record layer are reflected or scattered at the scratched portion.
As already stated, the information held by the optical disk is not recorded on the readout surface but in the information record layer beneath the surface. Therefore, a scratch on the readout surface does not directly damage the information. Accordingly, if the information record layer is safe, it is possible to read the information once more by removing the scratch from the readout surface.
The principle of restoring an optical disk is hereinafter described using FIGS. 9 and 10. FIG. 9 is an enlarged sectional view of a single-layer BD with the readout surface directed upwards, in which an information record layer 130 formed on a substrate 120 made of a polycarbonate resin or similar materials having a thickness of approximately 1.1 mm is covered by a cover layer 140 of approximately 0.1 mm in thickness and a hard-coat layer 150 of 2 to 5 micrometers in thickness.
FIGS. 10(a) and 10(b) are both an enlarged sectional view of an optical disk with the readout surface directed upwards, showing the portion corresponding to the area surrounded by the circle A in FIG. 9. FIG. 10(a) shows the readout surface with scratches 160, and FIG. 10(b) shows the same portion with the scratch ground away.
In this manner, the optical disk can be restored by grinding away the disk surface by a thickness approximately equal to the depth of the scratch and then performing a mirror-polishing process.
Optical disk restoration apparatuses for polishing an optical disk to remove a scratch from its readout surface in the previously described manner have been conventionally known (for example, refer to Patent Document 1). An example of the conventional optical disk restoration apparatuses includes a rotary table, on which an optical disk to be restored can be set, a disk-shaped polishing body, and other components. This apparatus polishes the readout surface of the optical disk by rotating the polishing body and the rotary table while maintaining the polishing body in contact with the readout surface.